https://phys.org/news/2018-07-anti-speciesism-veganism-basis-forarationalsociety.html

Singer's main contribution is to have redefined the identity of the moral community. He assumes that no human should be excluded from it. This therefore requires identifying a characteristic that is present in all humans without exception. For Singer, the capacity to suffer, or sensitivity, being equally shared between all humans, constitutes the only possible selective criterion. Since animals also have this ability, Singer proposes that they must necessarily be included in our moral sphere. In his 1975 book Animal Liberation, he wrote:

"There can be no reason – apart from the selfish desire to preserve the privileges of the exploiting group – to refuse to extend the fundamental principle of equal consideration of interests to members of other species."